The present invention relates to bursting discs, and particularly although not exclusively to graphite bursting discs of the type used in the pharmaceutical preparation and chemical industries.
It is well known in the chemical engineering and pharmaceutical industries to provide pressure relief devices for protecting pressure systems from over pressurization. One such pressure relief device is the known bursting disc, which comprises a substantially planar disc of graphite material which is installed at a suitable location in the pressurized system. When the pressure at one side of the disc rises above a predetermined design pressure, the graphite disc ruptures thereby releasing pressure from the system. One such typical application is on reaction vessels and chambers. In this case, the disc is attached to a flanged pipe outlet of a reaction chamber so that when the pressure in the reaction chamber rises above a predetermined design pressure, the graphite disc ruptures, releasing pressure in the reaction chamber.
Known bursting discs are of two types, the mono block type which is a singular item machined from a solid block of graphite and which has a peripheral outer ring 2 of graphite, and a thinner graphite membrane 4 extending across the ring, as shown in cut away bisected view in FIG. 1 of the accompanying drawings. Another type is the-replacement element type bursting disc which comprises a circular graphite membrane in the form of a replaceable wafer disc, and a two part holder device into which the graphite membrane is arranged to fit. A cut away bisected view of a typical replacement type membrane is shown in FIG. 2 and comprises the membrane 6 and a ring gasket 8 on either side thereof.
Either type is installed between pipework flanges. When the membrane ruptures or fractures, it relieves pressure from the reaction chamber or other pressurized installation.
In each type in order to create a differential pressure between one side of the bursting disc which faces the inside of the reaction chamber, and the other side of the bursting disc, which typically may lead to a venting pipe, the disc must create a gas impermeable barrier. As the graphite in its basic form is porous, to produce an effective bursting disc it is necessary to modify the graphite to make the membrane impermeable. If the graphite is porous, the response time of the disc to changes in differential pressure on opposite sides of the disc is poor, and the disc may fail to burst at all, or not burst quickly or at the correct pressure.
Conventionally, the graphite is impregnated with a thermo-setting resin, such as phenolics, PTFE polytetrafouorethylene emulsions or a furanic resin.
Impregnation with such resins creates an impermeable bursting response as soon as the fifferential pressure between sides reaches a predetermined design pressure.
However, the resin impregnated graphite bursting disc is limited in operating temperature by the temperature at which the phenolic and furanic resins dissociate from the graphite. Typically such dissociation occurs at around 200xc2x0 C. and consequently, for constant operation of reactions at greater than 200xc2x0 C., known graphite bursting discs are unreliable. At such temperatures, small pores of resin dissociate from the graphite and the graphite becomes porous leading to reduced bursting performance, and contamination of the reaction chamber with resin debris.
Typically, known graphite bursting discs are used for reactions which occur at temperatures up to 165xc2x0 C.
It is also known to provide impregnated graphite bursting discs with a polytetrafluoroethylene (PTFE) coating or fluoropolymere liner to provide additional corrosion protection. However, such coated or lined impregnated discs are limited in operating temperature due to the impregnated graphite as described above and are only effective for use at temperatures up to 165xc2x0 C. Furthermore the coatings are porous and not impermeable to gas so that their use is only effective with impregnated or non-porous a graphite
The inventors of the present invention have found experimentally, that improvements to a graphite bursting membrane are possible, which increase the maximum steady state operating temperature at which a graphite bursting membrane can reliably function.
The inventors have also found that improvements to a graphite bursting disc are possible which improve the corrosion resistance compared to the prior art thermo-setting resin impregnated graphite bursting discs.
According to one aspect of the present invention there is provided a pressure relief device comprising an unimpregnated graphite membrane having a gas impermeable coating.
Preferably the coating is a substantially non-micro porous coating.
The coating is preferably a surface coating bonded to the membrane.
The coating preferably comprises a polymer.
The coating may comprise a fluoropolymer.
The coating may comprise a fluoroalkyne.
The coating may comprise TEFLON(copyright) or polytetrafluoroethylene.
According to another aspect of the present invention there is provided a pressure relief device comprising an unimpregnated graphite membrane incorporating a fluoropolymer.
Preferably the membrane comprises a fluoropolymer surface coating.
The fluoropolymer may comprise TEFLON(copyright).
The membrane may comprise a porous graphite membrane having a fluoropolymer surface coating.
Unlike the prior art devices, the graphite membrane of the device of the invention is not impregnated with a resin.
Preferably the coating is sintered to the graphite disc.
Preferably the coating is of thickness in the range 5 microns to 300 microns. The thickness may be in the range 25 to 125 microns.
The coating may be applied to one or more surfaces of the graphite membrane. The whole surface of the graphite membrane may be coated or one or more sides and edges of the membrane may be coated. In particular, the coating may be applied to the pressure side of the membrane only. This is equally effective and may reduce manufacturing costs and improve performance. Furthermore the disc may be machined to give the desired size or shape and to meet required tolerances after th coating process by machining or grinding the uncoated surfaces thereof.
Preferably the fluoropolymer coating is substantially holiday free.
Sintering the coating may have the effect of homogenising the fluoropolymer, and removing bubbles or micro cavities in the fluoropolymer, and thereby removing any holidays in the coating. By sintering the coating, the fluoropolymer coating may be made impermeable, and thereby promote pressure differential between opposite sides of the membrane, enabling an accurate bursting pressure to be designed for the membrane, even if the body of the membrane remains porous.
Preferably the fluoropolymer coating and graphite disc are sintered at temperatures in the range 300-600xc2x0 C., and suitably in the range 350-550xc2x0 C.
The invention includes a method of manufacturing a fluoropolymer coated graphite membrane comprising:
coating an unimpregnated graphite membrane with a fluoropolymer in fluid form; and
sintering the fluoropolymer coated graphite membrane.
The membrane may be machined or further machined to the desired size or shape either before or after the coating and/or sintering step.
Preferably said step of sintering comprises raising the temperature of the graphite membrane and fluoropolymer coating in the range 300-600xc2x0 C., and suitably in the range 350xc2x0 C. to 550xc2x0 C. for a predetermined period.
A bursting disc incorporating such a membrane may be of the mono block type or of the replacement element type.
A fluoropolymer coated unimpregnated graphite bursting disc may have a steady state operating temperature in excess of 200xc2x0 C. Further, as the fluoropolymer coating may have non stick properties, bursting discs made from the coated membrane may find new applications for example in industries, where low contamination of product is an important criteria.
Since graphite is typically manufactured at some 3000xc2x0 C., the only effective temperature limitation on a device of the present invention is the coating temperature unlike for impregnated graphite membranes which are limited in operating temperature to 165xc2x0 C.
Apart from they improved operating temperature, the device of the invention has improved corrosion resistance and is more cost and time effective to produce since the process of impregnating the graphite is avoided.
Furthermore, the process of impregnation with for example furanic or phenolic resins renders the porous graphite impermeable. A porosity test can be performed to ensure the impregnated membrane complies with required standards. However, if a fluoropolymer coating is applied to the impregnated graphite, a porosity test will not indicate whether the coating is holiday free.
With the present invention however, a porosity test performed on the coated membrane gives a reliable indication of whether the fluoropolymer coating is holiday free since the graphite itself is unimpregnated and therefore porous. Thus, when a porosity test is completed successfully the fluoropolymer coating is holiday free.
The use of a non-micro porous fluoropolymer coating, produced for example by sintering, in an embodiment of the present invention gives an additional major advantage by providing the gas impermeable coating. Known fluoropolymer coatings applied to impregnated graphite are micro-porous so that their use is only effective with impregnated graphite which itself is non-porous.